When an electric field is applied to a certain type of electro-optical material having ferroelectricity and translucency, the refractive index is changed to generate an electro-optical effect. Therefore, such an electro-optical material has been expected to be used as an optical component such as an optical switch or an optical shutter.
Hitherto, a translucent single crystal of LiTaO3, (Sr, Ba)Nb2O6, or the like has been widely used as such an electro-optical material. However, single crystals are disadvantageous in that the production cost is high, and that it is difficult to obtain materials having a large size.
Accordingly, use of a translucent polycrystalline ceramic has recently, been studied. Such a translucent polycrystalline ceramic (hereinafter, the term “translucent ceramic” used in this description means a translucent polycrystalline ceramic) is obtained by compacting a raw material powder, sintering the resulting compact, and polishing the sintered compact. Therefore, the production cost can be lower than that in a case of a single crystal. (Pb, La) (Ti, Zr)O3 (hereinafter referred to as “PLZT”) is known as a typical example of such a translucent ceramic. In this PLZT, some Pb atoms of Pb(Ti, Zr) O3, which is a ferroelectric material, are replaced with La atoms, thereby somewhat decreasing ferroelectricity, and translucency is provided by further performing hot-pressing or the like.
However, PLZT contains lead, and thus the use of PLZT is not preferable from an environmental viewpoint. Furthermore, PLZT is disadvantageous in its optical characteristics become unstable because of volatilization of lead.
Consequently, strontium barium niobate ((Sr, Ba)Nb2O6) translucent ceramics which have a tungsten bronze structure have been actively developed as a lead-free electro-optical material.
For example, Patent Document 1 proposes a method of producing a translucent sintered compact including preparing a mixed solution of aqueous solutions of oxalates of Sr, Ba, and Nb, the mixed solution having a composition ratio represented by a general formula Sr1-xBaxNb2O6 (wherein 0.25≦x≦0.75), allowing ammonia to interact with the mixed aqueous solution to produce a precipitate, thermally decomposing the precipitate, compacting the resulting product, and sintering the resulting compact in an oxygen atmosphere.
Furthermore, Patent Document 2 proposes a method of producing a sintered compact of (Sr, Ba)Nb2O6 including preparing a mixed solution containing Nb, Sr, and Ba, performing a hydrolysis reaction to produce a sol, drying the sol, calcining the dried sol at 800° C. to 1,400° C., mixing the resulting calcined product with a Nb compound, calcining again the mixture at 800° C. to 1,400° C. to prepare a calcined powder, compacting the calcined powder, and sintering the compact at 1,100° C. to 1,400° C.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 62-171957
Patent Document 2: Japanese Unexamined Patent Application Publication No. 63-291844